Mineral exploration in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands

This report describes an exploration programme for gold and base metals in the Precambrian Lewisian terrain of north-west Scotland. Supracrustal rocks in the Lewisian have several of the characteristics of ‘greenstone belts’ in basement gneiss terrain which host many of the world’s gold deposits. Often these deposits are closely associated with banded iron formations (BIFs) and iron-rich exhalites are present in the Gairloch and Loch Maree belts associated with a mixed volcanosedimentary succession. The exhalites include massive and microbanded iron oxide facies, together with various banded silicate and sulphide facies rocks which form well-defined and laterally extensive suites. Although previously recognised at the Kerry Road copper-zinc-gold deposit near Gairloch, these lithologies had not, until the publication of MEW reports on Glenelg and Scardroy, been the subjects of extensive study as potential hosts for base- or precious-metal mineralisation. Selective lithogeochemical sampling of prospective areas (Loch Maree, Flowerdale, Melvaig, Tiree, Coil, Uist, Benbecula and the Central Belt) was used as the preferred exploration medium. In addition to providing information on mineralisation styles, these data provided a means for correlation of the supracrustals between different areas. Lithogeochemical mapping was supported by ground geophysics, principally total field magnetics and VLF, which provided detail on the disposition and extent of potentially mineralised exhalative suites where concealed by glacial drift. Petrological investigations provided additional information on the complex primary mineral compositions and alteration assemblages at these localities. Data listings, together with detailed logs of samples, are available on request from the Minerals Programme, Minerals Group, BGS, Keyworth. The tectono-stratigraphy of the Loch Maree and Flowerdale Forest areas has been clarified and can now be compared with the better known succession in the Gairloch area. In particular, the successions at Loch Maree and Gairloch have been correlated, and the chlorite schist at Abhainn na Fuirneis is thought to be comparable to that at Gairloch which forms the footwall to the Kerry Road deposit. This should provide a better framework to guide more detailed exploration of the supracrustal succession and its contained stratiform mineralisation. Detailed exploration of the rocks adjacent to chlorite schist at Abhainn na Fuirneis and the quartz-magnetite schist further to the north-east with 0.7 ppm gold is recommended The succession at Loch na Beinne (in the Flowerdale Forest area) is similar to that at Kerry Road and the presence of sulphide-bearing and banded iron formations with significant gold values (up to 4 g/t Au) is highly prospective. It is strongly recommended that detailed exploration should concentrate on the Lewisian concealed beneath a shallow cover of Torridonian rocks near Loch na Beinne and northwest along strike for 8 km. Reconnaissance traverses using ground magnetic and VLF-EM techniques have been successful in establishing the presence at several localities of high-amplitude anomalies of limited strike extent. At Loch na Beinne these anomalies are coincident with the exposure of banded oxide and sulphide facies exhalites containing up to 4 g/t gold. Similar methods should be applied to targets in the Lewisian beneath the shallow sedimentary cover and along the 8 km of concealed strike length. There is potential within this strike length for the discovery of a Be&i-style copper-zinc-gold deposit similar to that at Kerry Road.